Not Applicable.
Not Applicable.
The present invention relates in general to permanent magnet alternating current (PMAC) motor controls, and, more specifically, to a digital motor current controller for a permanent magnet AC motor having reduced bandwidth variations caused by temperature changes.
Of all the physical environments where electric motors are used, the automotive vehicle environment is one of the harshest. Operating temperatures for non-engine components can range from very cold to very hot (e.g., from about xe2x88x9220xc2x0 C. to about +50xc2x0 C.). An electric motor used in this environment must operate within its specified performance over this full temperature range.
Electric power assist steering systems are well known in the art. These systems often utilize a rack and pinion gear set to provide power assist by using an electric motor to either (i) apply rotary force to a steering shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver""s applied torque to the vehicle steering wheel, and (ii) sensed vehicle speed. The motor may comprise a PMAC motor, a brush-type motor, or a variable reluctance motor depending upon the application and its requirements.
Ideally, the electric motor and control system of an electric assist steering system will have a bandwidth greater than that of the steering system itself so that the response of the electric motor does not negatively impact the stability or performance of the steering system. A substantially constant bandwidth of the motor control current loop is desirable so as to achieve consistent performance over a larger range of motor conditions, especially with regard to temperature variations.
Resistance of the copper windings in a permanent magnet AC motor changes with temperature. The changed response has the undesirable effect of narrowing the closed loop bandwidth of the current control loop. Therefore, the controller may require compensation for temperature changes depending upon how much impact the reduction in bandwidth has on the system performance.
The present invention has the advantages of compensating for motor resistance variations with temperature to assure a consistent response of a PMAC motor for use in a harsh temperature environment. In particular, the motor resistance variations is compensated by tuning of the motor control current loop integral gain.
In one aspect of the invention, a motor current controller controls an instantaneous current flow in a permanent magnet AC motor via a switching bridge in response to a current command from a main motor controller, an instantaneous current value from a motor current sensor, and a temperature signal from a motor temperature sensor. The motor current controller comprises a difference element comparing the current command and the instantaneous current value to generate an error signal. A proportional-integral regulator generates a command signal for controlling the switching bridge in response to the error signal, wherein the proportional-integral regulator includes a proportional gain value and an integral gain value. A gain control circuit generates the integral gain value and loads the integral gain value into the proportional-integral regulator. The integral gain value is determined in response to the temperature signal so that a current loop comprising the motor, the switching bridge, and the proportional-integral regulator maintains a substantially constant loop bandwidth.